One of the most confusing things about learning
basic particle physics is mastering all of the particle names. I mean, you have fermions, bosons, baryons,
mesons, leptons, and on and on and ons. It’s really quite bewildering. So in this video, I thought I’d try to help
you sort it all out. To begin with, there are two types of names. There are the names that are specific types
of particles, like the electron, proton, or neutron. Then there are the names which are the classes
of particles- ones that distinguish between different kinds of spins, the forces the particles
feel, and the kinds and types of particles found inside them. To make things even more complicated, some
of the names were picked in the 1940s or something and the names are historical- meaning that
back in the day, the particles were classified into categories that made sense then, but
don’t anymore. So let’s talk about some of the obvious
ways in which particles can be characterized. They can be characterized by mass- into categories
that are small, medium, and large. They can be categorized into having different
kinds of spin- say having spins that are either an integer or half integer multiple of the
fundamental unit of spin. That just means that particles can have spins
of 0, 1, 2, and things like that, or a half, three halves, five halves, and so on. Then there is electrical charge, with particles
having either electrical charge or being neutral. It turns out that there isn’t a special
word distinguishing between the two, which is weird, but, well, that’s the way it is. And then one can classify the particles as
feeling the strong force or not. This was also historically important. So those are the biggies. Now let’s dig into the names. We’ll start with an obsolete set of categories. You’d think it’d be silly to talk about
an obsolete category, but they have historical significance and can cause modern confusion,
so I’m gonna do that. So, let’s think about the 1940s and 1950s. At that time, it seemed to make great sense
to classify particles in terms of their mass. Scientists kind of based the names with the
proton and neutron as being the standard. Protons and neutrons have about the same mass,
so, well, that kind of makes sense. There were the very light electrons and neutrinos,
with masses no bigger than 0.05% the mass of the proton. Because this was so small, these particles
were called leptons, for the Greek word leptos, which means fine or small or thin. That seemed like a great name for light particles. In contrast, there were particles that were
the mass of the proton or maybe twice as heavy. Because these were the giants of the particle
world, they were called baryons, for the Greek word barys, for “heavy.” Again- a sensible name. Finally, there were particles that had a mass
somewhere between the leptons and the baryons. Their masses were in the range of about ten
percent that of the proton to about half of a proton. These particles were called mesons, from the
Greek word mesos, which means “intermediate.” Briefly, the word “mesotron” was used,
but, well, that didn’t last very long. Now, this mass-based classification makes
sense, but it turns out to be a false lead. Baryons feel the strong force, while leptons
do not. Most of the mesons also feel the strong force,
but there was an outlier, called the muon. It was a low mass meson, but it didn’t feel
the strong force. That ended up being a powerful clue in the
proper way to classify particles. This strong force classification ended up
being important. There was a name for particles that felt the
strong force. They were called hadrons. This word actually was invented pretty late. P hysicist Lev Okun coined it in 1962 and
it comes from the Greek word adros, which also means heavy. On the spin side, there were the particles
with integer spin, which were called bosons after Satyendra Nath Bose, who developed the
mathematics describing how they behaved and worked with Einstein to get it published. And then there were the fermions. Fermions are particles with half integer spin
and were named after Enrico Fermi, who, along with Paul Dirac, pioneered the mathematics
that described them. The heavy baryons and the light leptons were
all fermions, while most of the middleweight mesons were integer-spin bosons. However, the muon was a fermion, which was
another clue that the muon didn’t fit into the historical and mass-based lepton, meson,
and baryon classifications. It was in the 1960s that Murray Gellman and
George Zweig independently came up with a key insight that made it possible to clarify
the whole thing. They realized that all of the baryons and
most of the mesons were made of smaller particles that we now call quarks. Quarks are fermions with spin 1/2, although
because the spins can point in one direction or the opposite, they can have spins of plus
1/2 or minus 1/2. Quarks feel the strong force, and, since the
muon didn’t feel the strong force, it was then clear that it actually belonged in the
lepton category, not the meson one. It also meant that the muon didn’t have
quarks inside it. So now we’re ready for the proper way of
characterizing particles. The spins of particles are properly separated
into the spin-half fermions and the integer spin bosons. Leptons contain no quarks and hadrons do. The mesons and baryons are both hadrons. Mesons contain a quark and an antimatter quark,
while baryons contain three quarks. Since quarks are fermions with spin 1/2 or
minus 1/2, you can see how they explain the spin of mesons and baryons. Take two particles with a spin of plus and
minus 1/2 and they have to add up to an integer spin, either -1, 0, or 1. That’s why mesons are integer-spin bosons. Now take quarks three at a time, and you have
to have half-integer spin particles, with spins of -3/2, -1/2, 1/2, or 3/2. That’s why baryons are spin-half fermions. Leptons aren’t thought to contain smaller
particles within them. Leptons are kind of like quarks, but without
feeling the strong force. So leptons are fermions. The diagram you see here is kind of a Venn
diagram of particles. There are the distinct fermion and boson classes. Within those classes, some particles feel
the strong force and some don’t. What makes them feel the strong force is they
carry quarks. There are bosons which don’t carry quarks. They are the photon, the W and Z bosons, the
gluon and the Higgs boson. All of them are particles that causes forces
to happen. The known forces are the strong force, the
weak force, electromagnetism and gravity. These bosons that aren’t hadrons are responsible
for these forces. There is one force-causing boson that hasn’t
been discovered, and that’s called the graviton, which is thought to be responsible for gravity. I predict it will be millennia before we find
direct evidence for gravitons. So that’s it. That’s how particles are characterized. All the other names- things like electrons,
protons, neutrons, neutrinos, pions, kaons, Delta particles, J/psi, et cetera, can all
be classified by this basic diagram. It’s also probably important to remember
that these particles I just mentioned are all examples of specific particles, not general
ones. For example, there’s only one type of electron. And even the less familiar particles- like
the muon and pion- are contractions of mu lepton and pi meson. Keeping separate in your mind the specific
particles and general ones is key to understanding this whole subject. For those of you who are adventurous, I’ve
posted a flow chart in the description of the video that helps you classify any subatomic
particle you discover. And, if you find a particle that the flow
chart doesn’t describe, let me be the first one to congratulate you on your upcoming trip
to Stockholm. I hope that this video helped you a little
figuring out the nomenclature of the hundreds of subatomic particles we’ve discovered. It’s still probably pretty confusing, but,
I promise- it all makes sense. It just takes some time to work it all out. If you liked this video, please like the channel
and share it with your friends- and, of course, feel free to tell us what you think about
it in the comments below. We love to hear from our viewers. The subatomic world is fun and exciting and
explains the universe around us- and that is why I always say- that physics is everything.
